Of the sunlight reaching the earth (including infrared light, visible light, and ultraviolet light), 5 to 6% is ultraviolet light. The ultraviolet light has short wavelengths, which are thus high-energy electromagnetic waves. Therefore, the ultraviolet light is known to decompose many kinds of materials and to cause quite some damages to a living body.
Therefore, ultraviolet shielding agents are used for protecting skin from inflammation or skin cancer due to the ultraviolet light by formulating them in the cosmetics, or they are mixed with paints to prevent a pigment from fading due to decomposition by ultraviolet light. In these cases, an unnatural skin whitening of the cosmetics and a color change of paints can be prevented by increasing the transparency in the visible light region. Therefore, the ultraviolet light is desirably blocked while the transparency in the visible light region is maintained.
The ultraviolet shielding agent comprising organic compounds used as effective ingredients prevents the transmission of the ultraviolet light on account of the specific absorption of ultraviolet light by the organic compounds. For example, an ultraviolet absorbing composition comprising substituted N,N'-bis-aromatic formamidines is known (Japanese Patent Examined Publication No. 61-09993). However, the organic ultraviolet shielding agents have the problem that although the absorbed ultraviolet light is at the same time likely to act to decompose, with the result of an undesirable lowering of the shielding ability over time. Regarding their application to cosmetics, the kinds and amounts of the ultraviolet shielding agents formulated are restricted owing to effects caused on human bodies, and thus it is difficult to achieve a good shielding performance within a controlled range. Furthermore, when they are used in high proportions, stickiness increases, resulting in an unpleasant feeling.
On the other hand, the ultraviolet shielding agent comprising an inorganic compound contains inorganic fine particles and prevents the transmission of ultraviolet light by the absorbing ability and the scattering ability of the composition. The inorganic ultraviolet shielding agent is superior to the organic ultraviolet shielding agent because the composition containing the inorganic ultraviolet shielding agent is not decomposed by the ultraviolet light with the passage of time and has little effects on the human bodies.
However, since the inorganic ultraviolet shielding agents are present in the form of particles, it is more difficult with inorganic ultraviolet shielding agents when compared with organic ultraviolet shielding agents to block the ultraviolet light while maintaining high transparency in the visible light region.
In order to exhibit an effective light shielding ability in the ultraviolet light region while maintaining high transparency in the visible light region (light wavelengths of from 400 to 800 nm), the composition has to be microgranulated to give ultrafine particles to be highly dispersed so as to increase the ultraviolet shielding ability (absorbing ability, scattering ability). However, in the case of using such ultrafine particles, problems may arise in the dispersion stability due to the aggregation of the ultrafine particles, and in the catalytic activities of the ultrafine particles.
In order to improve dispersibility, the ultrafine particle surfaces may be coated with other materials. For example, skin cosmetics comprising an oily cosmetic base material and a hydrophobic titanium oxide powder are known (Japanese Patent Examined Publication No. 59-15885). However, a suitable solvent has to be selected depending upon the properties of the coating layer formed on the surfaces. Also, since the particles are still ultrafine, the aggregation can only be lowered to a limited extent even if the surface treatment is conducted. In publications other than those mentioned above, there have been known cosmetics containing a powder obtainable by coating titanium oxide with a particular amount of mixed hydrates comprising silicate hydrates and alumina hydrates, wherein the titanium oxide is nearly spherical or irregularly shaped and having an average particle size of from 30 to 70 nm, and further by coating the surfaces with a silicone oil (Japanese Patent Laid-Open No. 2-247109). However, since in this publication the above powder is obtained by drying and pulverization of the product obtained after coating with the mixed hydrates comprising silicate hydrates and alumina hydrates or after coating the surfaces of the powder with the silicone oil, it is extremely difficult to pulverize the titanium oxide ultrafine particles to the size of the primary particles, namely the titanium oxide ultrafine particles are aggregated and have a large particle size, so that the transparency and the ultraviolet shielding ability of the above obtained powder are lowered. Such technological problems arise in maintaining the dispersibility of the ultrafine particles stable. Therefore, it is increasingly significant to find a way to achieve a high dispersibility of the ultrafine particles and maintain it at that level.
Also, for the purposes of solving the problem of a difficult handling of the metal oxide ultrafine particle powder and of easily providing cosmetics comprising uniformly dispersed ultrafine particles, starting materials for cosmetics comprising metal oxide ultrafine particles having a particle size of not more than 0.1 .mu.m, a dispersion medium, and a dispersant, wherein the content of the ultrafine particles is not less than 10% by weight, have been known (Japanese Patent Laid-Open No. 6-239728). However, although the aggregation of the ultrafine particles and the deterioration of the dispersant, the dispersion medium, and cosmetics base materials caused by the catalytic activities of the metal oxide ultrafine particles are the problems to be solved, they have not been considered in this publication. Moreover, the content of the metal oxide ultrafine particles in the starting materials for cosmetics is limited to not less than 10% by weight by considering the amounts formulated in the cosmetics. However, as long as the metal oxide ultrafine particles are uniformly and stably dispersed, the function of the metal oxide ultrafine particles is high, and the content of the metal oxide ultrafine particles in the starting materials for cosmetics needs not be limited to an amount of 10% by weight or more for all kinds of cosmetics.
Therefore, in order not to lower the ultraviolet scattering ability by the aggregation of the inorganic ultrafine particles, composites of the inorganic ultrafine particles are often formed with other relatively large carrier particles. For example, a thin flaky material dispersed with metal compound fine particles is known (Japanese Patent Laid-Open No. 63-126818). However, this publication never discloses a specific construction of the fine particles for improving both the shielding ability of the ultraviolet light and the transparency of the visible light.
On the other hand, composite fine particles comprising ultrafine particles dispersed in and supported by the solid material are proposed. Conventional ultraviolet shielding composite fine particles include, for example, a composite powder in which a fine particle powder, such as TiO.sub.2 l is uniformly dispersed in plate particles of metal oxides, such as SiO.sub.2 (Japanese Patent Laid-Open No. 1-143821); and composite particles in which a zirconium oxide powder or an aluminum oxide powder is carried on a surface of the matrix particles comprising such materials as nylon resins, silicone resins, and silicon oxide, wherein a titanium oxide powder or a zinc oxide powder is dispersed in an inner portion of the matrix particles (Japanese Patent Laid-Open No. 2-49717).
However, in order to use the above composite particles as ultraviolet shielding agents, the composite particles have to be usually dispersed in a medium in the actual environment. In this case, since the metal oxides, such as titanium oxide, contained in those composite particles have catalytic activities, the deterioration of the medium is likely to take place. Also, when the difference between the refractive index of the composite particles and the refractive index of the medium is large, light scattering takes place at an interface of the composite particles and the medium, thereby making both the transparency in the visible light region and the shielding ability in the ultraviolet light region poor. Although these problems need to be solved, they have not been considered in the above publications.
Therefore, in order to suppress the catalytic activities of the ultrafine particles, methods for coating a surface of the ultrafine particles with various materials have been used. For example, cosmetics comprising fine particle powder of titanium hydroxide which is obtained by hydrolysis of a titanium alkoxide is known, wherein a basic compound and a hydrocarbon compound having a boiling point of from 100 to 200.degree. C. and/or a silicone having a particular molecular structure are added during the production (Japanese Patent Laid-Open No. 5-70331). However, in order to produce titanium hydroxide fine particle powder, the production process must undergo drying and pulverization processes, which results in a large particle size of the obtained titanium hydroxide fine particles. Thus, it is difficult for the particles to be highly exhibit scattering properties of ultraviolet light B (light wavelengths of from 280 to 320 nm), while maintaining high transparency in the visible light region. Also, the above publication has neither considered nor disclosed any method for producing ultrafine particles for the titanium hydroxide particles or methods for dispersing the ultrafine titanium hydroxide particles in cosmetics, in order to satisfy both high transparency in the visible light region and high shielding ability to the ultraviolet light. Moreover, the ultraviolet shielding material disclosed in this publication is considered to be effective, by titanium hydroxide or titanium oxide, in absorbing the ultraviolet light B (light wavelengths of from 280 to 320 nm), the ultraviolet light B only penetrating the epidermis and a relatively upper layer of the dermis and causing sunburn or skin cancers. However, they are not at all effective in absorbing the ultraviolet light A (light wavelengths of from 320 to 400 nm), especially the light having light wavelengths of from 350 to 400 nm, which are wavelengths close to those of the visible light, the ultraviolet light A reaching skin layers beyond the dermis and producing suntan or fibrous denaturalization in the dermis. In other words, the ultraviolet absorbents disclosed in this publication mainly exhibit absorption of the ultraviolet light B by titanium hydroxide or titanium oxide, but the exhibition of their ultraviolet absorption effects are limited to a light wavelength of up to about 300 nm for an anatase-type titanium oxide and to a light wavelength of up to about 320 nm for a rutile-type titanium oxide.
Of the ultraviolet light reaching the earth, the energy proportion of the ultraviolet light A is about 15 times that of the ultraviolet light B. Therefore, in view of the above energy proportion of the ultraviolet lights A and B, it is important to shield not only the ultraviolet light B but also the ultraviolet light A, rather than shielding the ultraviolet light B. Moreover, it is becoming increasingly important to shield both the ultraviolet light B and the ultraviolet light A, while maintaining a high transparency in the visible light region. In particular, in the case where the ultraviolet light A is shielded, it is important to shield light wavelengths of the ultraviolet light of from 350 to 400 nm, which are closer to light wavelengths of the visible light.
As mentioned above, in order to exhibit an effective shielding ability in the ultraviolet light region while maintaining and high transparency in the visible light region, the ultraviolet shielding materials are made ultrafine to be present in a highly dispersed state. In order to further improve the transparency in the visible light region, it is important to keep the difference between the refractive indices of the ultraviolet shielding materials and those of surrounding media small. In the titania-containing composite powders disclosed in Japanese Patent Laid-Open Nos. 1-143821 and 6-116119, the refractive index of the composite powder is determined by the compositional ratios of the components, and, therefore, it is inevitably limited to match the refractive index of the composite powder to the dispersion medium having a given refractive index value. Therefore, great problems have been encountered in controlling the refractive index of the composite powder so as to match it with the refractive index of the dispersion medium used. An effective solution for solving these problems has been in great demand.
Further, in metal compound-containing porous silica bead, the production method thereof, and the powder deodorant produced (Japanese Patent Laid-Open No. 4-65312), fine particles of metal compounds having a primary particle size of from 0.001 to 0.3 .mu.m are contained in the particles of porous silica beads in an amount of from 0.1 to 30% by weight, and the porous silica bead contain substantially no voids of not less than 0.3 .mu.m. In this case, when the fine particles of the metal compounds contained therein are suitably selected so as to have a refractive index close to the refractive index of silica (the refractive index being in the range of from 1.4 to 2.0), silica particles with further improved transparency can be obtained. However, only defined are the ranges for the refractive index of the metal compound fine particles contained in the inner portion of the composite particle, and the refractive index of the overall composite particles is not defined.
As explained above, in order to solve the problems inherent in the ultraviolet shielding agents comprising the ultrafine particles, several attempts have been made to utilize composites mainly comprising metal oxides. However, many of the compounds exhibiting good ultraviolet absorption properties, such as TiO.sub.2 and ZnO, have relatively high refractive indices, so that the composite fine particles comprising composites of these ultrafine particles have refractive indices notably higher than aqueous solutions, conventional organic solvents, or polymers. When the above composite fine particles are dispersed in a medium, light scattering in the visible light region takes place at the interface of the composite fine particles and the medium, whereby the transparency of the composition is drastically lowered. However, there has been conventionally no technical ideas of controlling the refractive index of the ultraviolet shielding particle of the composite fine particles.
In the fields of resin fillers, fluorine-based inorganic compounds, such as MgF.sub.2 and CaF.sub.2, or fluorine-based organic polymers, such as polyethylene tetrafluoride, which are known as low-refractive index materials having high transparency, are added to powders, etc. as starting materials to lower their refractive indices.
For instance, Japanese Patent Laid-Open No. 4-85346 discloses a glass powder, used as a transparent inorganic powder for resin fillers, comprising metal oxides, such as SiO.sub.2, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, BaO, SrO, ZnO, and MgO, and metal fluorides, the glass powder having a refractive index (Nd) adjusted in the range of from 1.44 to 1.70. The publication discloses that since the glass powder has a high light transmittance and does not show strong alkalinity, the resins do not undergo any substantial denaturalization, and have significantly low instability in resin hardening. However, the publication merely discloses that a highly transparent inorganic powder for resin fillers is obtainable by changing the compositional ratio of the materials, and the above metal oxides, etc. are not present in the state of particles in the final product powder owing to the high-temperature melting production process, and this publication never mentions about the ultraviolet shielding ability. Further, this publication does not at all disclose that the composite fine particles comprising aggregates of two or more kinds of fine particles as in the present invention have the compositional dependency with respect to an average refractive index of the composite fine particles, nor does it have any such technical ideas.
In order to solve various problems inherent in the ultraviolet shielding agents mentioned above, the present inventors have previously developed ultraviolet shielding composite fine particles, method for producing the same, and cosmetics (WO 95/09895 Publication). In the publication, an ultraviolet shielding agent having high transparency in the visible light and high ultraviolet shielding ability was developed by combining fine particles (daughter particles) having the ultraviolet shielding ability with fine particle aggregates (matrix particles) in which the daughter particles are dispersed and incorporated, and by determining combinations of the two components according to the difference of their band gap energies, and thereby the optical properties of the ultrafine particles can be optimized. This ultraviolet shielding agent is characterized, besides the features mentioned above, in that, since the refractive indices can be controlled in a wide range by changing the materials of matrix/daughter particles and proportions thereof, high transparency can be exhibited even when dispersed in various media, and high transparency can be exhibited regardless of their shapes. Also, easy handling (transportation, surface treatment, formulation, etc.) is achieved owing to the size of the level of fine particles, and they are usable for cosmetics since they do not change the color tone.
However, this ultraviolet shielding agent is yet to be improved in the following:
(1) In the case where a particularly high ultraviolet shielding ability is desired in the use for cosmetics, the amount of the ultraviolet shielding agent has to be made large. In this case, an upper limit of the amount of the ultraviolet shielding agent has to be set so as not to impair the skin texture of the cosmetics due to the texture of the composite fine particle powder itself. PA0 (2) Also, in the case of using the ultraviolet shielding agent for cosmetics, the catalytic activities of ultrafine particles located near the surfaces of the composite fine particles have to be suppressed. In general, the suppression of the catalytic activities may be achieved by coating the surfaces of the composite fine particles with an inorganic material having substantially no catalytic activities. However, on the other hand, when coated with an inorganic material, the proportion of the daughter particles in the coated composite fine particles is lowered, which in turn may cause a problem that the ultraviolet shielding ability per unit weight of the composite fine particles comprising a surface coat may be lowered depending upon the thickness of the surface coat. PA0 (3) Further, the composite fine particles disclosed in WO 95/09895 are obtainable in the powder form by drying droplets of the starting material liquid mixture comprising the matrix particles and the daughter particles, and by thermal decomposition. Even when the above powdery composite fine particles are surface-coated with inorganic materials, etc., only the surfaces of the composite fine particles are coated. Therefore, when the composite fine particles undergo pulverization or disintegration in various applications, the uncoated inner portions of the composite fine particles are liable to be exposed, so that the suppression of the catalytic activities is liable to be insufficient. PA0 (1) Regarding the problem of causing the limitation of the ultraviolet shielding ability owing to the compositional upper limit of the composite fine particles, the texture of the powder can be lowered by having an even smaller average particle size of the ultraviolet shielding composite fine particles, and the compositional upper limit of the composite fine particles can be increased, thereby succeeding in enjoying a remarkably wider compositional degree of freedom. In this case, although the method disclosed in WO 95/09895 Publication is suitable for the production of the composite fine particles having a relatively large average particle size, it is not suitable for efficiently producing composite fine particles with a very fine size of a level of about 0.5 .mu.m or less. Therefore, the present inventors have found out a novel production method comprising the steps of preparing a liquid mixture containing a mixture of the matrix particle starting material and the daughter particle starting material; subjecting the liquid mixture to a mill treatment and/or a high-pressure dispersion treatment, to form composite fine particles in a liquid phase comprising aggregates of daughter particles/matrix particles. By using this method, the average particle size of the composite fine particles can be easily made even smaller, so that the skin texture of the powder can be lowered and the compositional upper limit of the composite fine particles can be increased, thereby succeeding in enjoying a remarkably wider compositional degree of freedom. PA0 (2) Further, in order to suppress the catalytic activities of the composite fine particles, the present inventors have found that instead of coating the surfaces of the composite fine particles with the inorganic material having substantially no catalytic activities, the surfaces are directly coated with a silicone which is a surface treatment agent giving a water-repellent ability, by which the catalytic activities of the composite fine particles can be unexpectedly substantially suppressed. Therefore, by coating the surface of the composite fine particles with a silicone, the surrounding medium of the composite fine particles is for practical purposes not deteriorated by the catalytic activities or the photocatalytic activities. Also, by coating the surfaces with the silicone, an oily medium and the surfaces of the composite fine particles become more compatible, and by the steric hindrance by the silicone, the dispersibility of the composite fine particles in the oil becomes good, so that the high transparency of the visible light and the high shielding ability of the ultraviolet light are liable to be easily exhibited. Also, since no surface coatings are formed by inorganic materials, the proportion of the daughter particles in the composite fine particles is kept high, thereby resulting in an increase in the ultraviolet shielding ability per unit weight of the composite fine particles. PA0 (3) Furthermore, the present inventors have found a method for carrying out a silicone coating onto the composite fine particles in a liquid phase. In this case, since a coating treatment with a silicone is carried out while maintaining a dispersion state in a liquid phase of the composite fine particles formed in a liquid phase, the silicone is assumed to be present not only on the surface of the fine composite particles but also in the inner portion of the composite fine particles, so that the surface of the primary particles, such as daughter particles, are presumably coated to some degree. Therefore, even when the composite fine particles coated with the silicone are subjected to disintegration, the resulting particles are substantially coated with the silicone, so that the suppression of the catalytic activities is not liable to be lost. PA0 (1) Ultraviolet shielding composite fine particles having transparency in a visible light region, comprising matrix particles comprising an aggregate of primary particles having an average particle size of from 0.001 to 0.3 .mu.m, the aggregate being formed while the primary particles retain their shapes; and daughter particles having an average particle size of from 0.001 to 0.1 .mu.m, the daughter particles being dispersed in and supported by the matrix particles, wherein the daughter particles have a smaller band gap energy than the particles constituting the matrix particles and are capable of absorbing ultraviolet light, and wherein the surfaces of the composite fine particles are coated with one or more silicones selected from the group consisting of modified silicones, reactive silicones, and silicone-modified copolymers, and wherein the ultraviolet shielding composite fine particles have substantially no catalytic activities; PA0 (2) An oil dispersion of the ultraviolet shielding composite fine particles as defined in item (1) above, obtainable by the steps of: PA0 (3) A method for producing ultraviolet shielding composite fine particles comprising matrix particles and daughter particles, the daughter particles being dispersed in and supported by the matrix particles, the ultraviolet shielding composite fine particles having substantially no catalytic activities and having transparency in a visible light region, the method comprising the steps of: PA0 (4) Cosmetics comprising ultraviolet shielding composite fine particles as defined in item (1) or item (2).